It is known to be desirable to measure a wide range of pressures using a single pressure transducer device. It is also known that piezoresistive pressure transducers adapted to measure relatively large pressures disadvantageously suffer from a relatively poor resolution or sensitivity when measuring relatively low pressures. That is, as the span of a sensor increases, the resolution or sensitivity of that sensor at the low end of the span decreases. An example of such a piezoresistive sensor is taught in commonly assigned U.S. Pat. No. 5,614,678, entitled “HIGH PRESSURE PIEZORESISTIVE SENSOR” and issued Mar. 25, 1997, the entire disclosure of which is hereby incorporated by reference. The reference also includes in the art cited, many other patents concerning pressure transducers to the assignee hereof.
Referring to FIGS. 4A-4C, the first steps in fabricating a piezoresistive pressure transducer according to the '678 patent are depicted therein. The details of these processing steps are described in commonly assigned U.S. Pat. No. 5,286,671 entitled “FUSION BONDING TECHNIQUE FOR USE IN FABRICATING SEMICONDUCTOR DEVICES”, the entire disclosure of which is also incorporated herein by reference. Referring first to FIG. 4A, a pattern wafer 40, which may be made of a single crystal semiconducting material 44 such as N-type silicon, is selected. Such wafers are commercially available and are well known in the art. The wafer 40 has high conductivity P+ (or P++) silicon areas 42 which have been created by diffusion using oxide and/or nitride masking and photolithography for example. After the diffusion process, the surface of the wafer 40 is treated with a conductivity-selective etch which does not attack the P+ (or P++) areas, leaving them raised from the surface as shown in FIG. 4A.
Referring now to FIG. 4B, there is shown a carrier wafer 50, which will eventually form the diaphragm of the transducer. Semiconducting material 53 is lightly doped N- or P-type silicon. An oxide layer 52 is grown on a surface of the wafer 53 using any well known oxidation technique. A typical technique for providing an oxide layer on a silicon substrate is implemented by heating the wafer 50 to a temperature of between 1000°-1300° C. and passing oxygen over the surface of the substrate 53. The passivating oxide layer 52 in this case is silicon dioxide.
Referring now to FIG. 4C, the next step in the procedure is depicted. As shown therein, the pattern wafer 40 of FIG. 4A which contains the piezoresistive sensing elements 42 has been bonded to the carrier wafer 50 of FIG. 4B to form a composite wafer 55. The bonding process is performed in accordance with the preferred fusion bonding technique disclosed in the incorporated '671 patent. The technique described herein mimics that disclosed in the '671 patent and utilizes the earlier described P+(or P++) doped semiconducting material 42 of the pattern wafer 40 and the oxide layer 52 of the carrier wafer 50 as bonding layers. Typical bonding conditions which join the two wafers together are temperatures of between 900°-1000° C. and times of between 5 and 10 minutes.
Referring now to FIG. 4D, it can be seen that the N-type silicon layer of the pattern wafer 40 has been removed entirely down to the P+ (or P++) piezoresistive sensing elements 42 in a selective conductivity etching process which uses the oxide layer 52 of the carrier wafer 50 as an etch stop. Such selective conductivity etching processes are well known in the art and operate by means of etchants which selectively attack the low conductivity N-type material without etching or in any manner attacking the high conductivity P+ (or P++) layers. After this etching process, the raised pattern of P+ (or P++) piezoresistive sensing elements 42 is left bonded to the dielectrically isolating layer 52 of the carrier wafer 50.
Referring now to FIG. 4E, the next step in the procedure is depicted. The semiconducting material 53 of the carrier wafer 50 is preferably a single crystal (100) semiconductor material which may be etched on the side opposite the sensing elements 42 using an isotropic or anisotropic etching technique. Both isotropic and anisotropic etching techniques are commonly practiced, and familiar to those skilled in the art. The etching process forms an aperture 68, which defines the active 64 and non-active 54 diaphragm areas. The thickness or vertical dimension of the active diaphragm area 64 may be of any desired dimension depending upon the length of time that the etching process is allowed to take place. The aperture 68 is preferably etched such that some of the sensing elements 42 are positioned above the non-active or fixed diaphragm area 54, and others are positioned above the active or deflecting diaphragm area 64. Those sensing elements positioned above the non-deflecting diaphragm region are designated outer sensing elements 47, while those sensing elements positioned above the deflecting diaphragm region are designated inner sensing elements 48. The sensing elements 47, 48 are preferably electrically coupled together in a Wheatstone bridge configuration as is well understood.
Referring now to FIG. 4F, there is shown the completed high pressure piezoresistive pressure transducer device 60. The carrier wafer 50, with the etched out aperture region 68 is secured to a supporting member 66. The supporting member 66 may be fabricated from single crystal silicon or may be glass, for example. Of course, other suitable supporting materials can be used. The bonding of the supporting member 66 to the carrier wafer 50 may be accomplished by means of an anodic bonding technique such as the one described in U.S. Pat. No. 4,040,172 entitled “METHOD OF MANUFACTURING INTEGRAL TRANSDUCER ASSEMBLIES APPLYING BUILT IN PRESSURE LIMITING” issued to Anthony D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. The entire disclosure of the '172 patent is also incorporated herein by reference. The bond is typically formed by applying a high electrical voltage through the composite structure under low pressure and temperature, thus bonding the carrier wafer 50 to the supporting member 66 and completing the device. The central region 70 of the diaphragm area 64 and member 66 cooperatively serve as an overpressure stop when exposed to an overpressure which overly-deflects the active area 64 towards the support member 66.
As set forth though, such a fabricated transducer can suffer from the aforementioned drawbacks. Accordingly it is an object of the present invention to provide a single chip multiple range pressure transducer operable over a broad range of pressures and which provides a high degree of sensitivity when being subjected to relatively low pressures.